I have a report with a table with one group. In the group's Detail row I have a field that pulls an "Amount" field from the database. In the group's footer I want a subTotal field that shows the total of the "Amount" fields in the group. How can I do this?

Try dropping the amount field to the group row and reporting services should do an agregation by defauld, you should see something like =sum(field!fieldname.value)